1. Field of the Invention
The present invention relates generally to an ink discharging device. More specifically, the present invention relates to an ink jetting device to be used in an ink jet printer, wherein a conductive ink is directly energized to be heated to boil so that the ink is jetted out by an expansion pressure of the vaporized ink.
2. Description of the Prior Art
Recently, ink jet printers with ink jetting devices therein have been widely used as, for example, printers for office computers' outputs for reasons of, such as, silence or less noisiness of their printing operations.
U.S. Pat. No. 3,179,042 discloses one of such ink jetting devices, in which a conductive ink is directly energized to vaporize by means of its own Joule heat so that an expansion pressure of the vaporized ink gas forces a portion of the conductive ink to be injected as ink droplets onto a print area of a paper to be printed upon. FIG. 1 is a sectional view of the ink jetting device which is substantially the same as that disclosed in the above-noted U.S. Patent. In FIG. 1, a nozzle 1 includes a tube 2 which is filled with a conductive ink 3 supplied from an ink tank or supply 4. The tube 2 is opened at its upper end to form a nozzle hole or opening. A pair of electrodes 5 and 6 with a predetermined gap therebetween are arranged below a level of the ink 3 within the tube 2. The electrodes 5 and 6 are connected to a power supply 7. When a switch 8 is closed, a voltage is applied across the electrodes 5 and 6. A reference numeral 9 designates a paper to be printed upon, and a reference numeral 10 designates droplets of the ink 3 propelled out from the nozzle hole to form a dot upon the print area of the paper 9.
Operations of the ink letting device as described above are as follows:
When the switch 8 is closed to apply the voltage across the electrodes 5 and 6, a current flow is generated through the conductive ink 3 contained in the gap between the electrodes 5 and 6 so that the Joule heat is produced. As a result, a portion of the energized ink 3 is vaporized into the form of bubbles which expand to exert a high pressure or force upon the ink above the gap so that the ink droplets 10 are propelled out from the nozzle hole onto the print area of the paper 9. In practice, there are multiple sets of nozzles 1, each having a corresponding pair of the electrodes 5 and 6 and the switch 8. By operating the switches 8 to select the particular pair or pairs of the electrodes 5 and 6 to be applied with the voltage, a desired letter or character can be printed on the paper 9 by a plurality of dots each formed by the ink droplets 10 jetted from the corresponding nozzle 1.
The prior art ink jetting device as described above, however, has the following problem:
When a large number of the nozzles 1 are provided, since each nozzle 1 must include its own pair of the electrodes 5 and 6, the number of the electrodes 5 and 6 inevitably amounts to quite large in total and thus the corresponding wirings also become complicated. As a result, the high densification of the nozzles which is essential for improving the printing characteristics, is very difficult to be realized.